1. Field of the Invention
The present invention relates to a projection optical system for use in a projection type image display apparatus such as a front projector and a rear projector.
2. Description of Related Art
FIG. 14 shows an exemplary reflective optical system proposed in Japanese Patent Application Laid-Open No. H08(1996)-292371. In FIG. 14, luminous flux from an object, not shown, passes through an aperture stop (AS) and is incident on a reflective optical element RE from a first surface R1. The luminous flux incident on the reflective optical element RE is refracted by the first surface R1, reflected by a second surface R2, a third surface R3, a fourth surface R4, a fifth surface R5, and a sixth surface R6, and refracted by a seventh surface R7 before it emerges from the reflective optical element RE. In this event, the luminous flux is primarily focused on an intermediate image forming surface near the second surface R2 and forms a pupil near the fifth surface R5.
The luminous flux emerging from the reflective optical element RE is finally focused on an image-pickup surface (an image-pickup surface of an image-pickup means such as a CCD or a CMOS sensor) IPS.
The optical system employs the optical element which has the plurality of curved or flat reflecting surfaces formed integrally to achieve a reduction in size of the overall reflective optical system. It is also possible to alleviate degraded optical performance due to low accuracy of arrangement of mirrors (assemble accuracy) which is often seen in a reflective optical system formed of mirrors arranged individually.
The optical system also has the aperture stop disposed closest to the object and is configured to focus the object image at least once within the optical system, thereby reducing the effective diameter of the reflective optical element while a wide field angle is provided. In addition, appropriate reflective power is provided for the plurality of reflecting surfaces constituting the optical element and the respective reflecting surfaces are decentered. Thus, the optical path in the optical system is bent in a desired shape to achieve a reduction in the overall length of the optical system.
Such a non-coaxial optical system is called an off-axial optical system. The off-axial optical system is defined as an optical system which, when the path of a central principal ray which passes from the center of the image and the center of the pupil is considered as a reference axis, includes a curved surface (an off-axial curved surface) to which the normal line at the intersection of the reference axis and the surface is not on the reference axis. The reference axis of the off-axial optical system is shaped to have turns.
In the off-axial optical system, constituent surfaces are typically non-coaxial and no vignetting occurs in the reflecting surfaces, so that it is easy to construct an optical system with reflecting surfaces. Japanese Patent Application Laid-Open No. H08(1996)-292372, Japanese Patent Application Laid-Open No. H09(1997)-222561, Japanese Patent Application Laid-Open No. H09(1997)-258105 and the like each have proposed a variable magnification optical system which employs such an optical element. Japanese Patent Application Laid-Open No. H09(1997)-5650 proposes a design method therefor.
Japanese Patent Application Laid-Open No. 2001-255462(corresponding to U.S. Patent Application Publication No. 2002-008853) has proposed application of the off-axial optical system to a projection optical system of a projection type image display apparatus. FIG. 15 shows the projection optical system proposed in Japanese Patent Application Laid-Open No. 2001-255462. In FIG. 15, L shows the projection optical system, PA shows an incident side reference axis of the projection optical system L, SA shows an emerging side reference axis of the projection optical system L, and θps shows an angle between the reference axes PA and SA.
In the projection optical system, luminous flux from a light valve LV is projected onto a screen, not shown, in a direction which is oblique to the screen.
In this manner, most projectors allow oblique projection to provide improved visibility for viewers. In other words, the angle θps in FIG. 15 generally is large to a certain degree.
When a projector is not only disposed on a desk but also disposed on a floor or suspended from a ceiling in use, a larger angle is required for the angle θps. A projection optical system for use in a rear projector or the like also achieves a reduction in thickness of the overall apparatus by projecting luminous flux onto a screen from behind in a direction which is oblique to the screen. As the angle of the projection with respect to the normal line to the screen is larger, the apparatus has a smaller thickness. The angle θps is also increased in this case.
As shown in FIG. 16, an optical system proposed in Japanese Patent Application Laid-Open No. H08(1996)-292371 or the like has reflecting surfaces arranged such that, when a counterclockwise direction is defined as positive, a reference axis is rotated in the positive direction at a first reflecting surface R101, in a negative direction at a second reflecting surface R102, in the positive direction at a third reflecting surface R103, and finally in the negative direction at a fourth reflecting surface R104. In short, the reflecting surfaces are disposed such that the angles between the reference axis and the respective reflecting surfaces are formed alternately in the order of positive, negative, positive, negative and so on.
When a small angle is formed between the incident side reference axis PA and the emerging side reference axis SA, no problem occurs in that arrangement of the surfaces. However, the arrangement has disadvantages if a rather large angle is formed between the incident side reference axis and the emerging side reference axis as in the projector.
FIG. 17(A) shows a plurality of reflecting surfaces arranged to direct the emerging side reference axis SA downward. As shown in FIG. 17(A), ξ1 represents an angle between the incident side reference axis PA and the reference axis from the first reflecting surface R101 to the second reflecting surface R102. An angle ξ3 between the reference axis from the second reflecting surface R102 to the third reflecting surface R103 and the reference axis from the third reflecting surface R103 to the fourth reflecting surface R104 is increased, while ξ2 between the reference axis from the first reflecting surface R101 to the second reflecting surface R102 and the reference axis from the second reflecting surface R102 to the third reflecting surface R103 is reduced, and ξ4 between the reference axis from the third reflecting surface R103 to the fourth reflecting surface R104 and the emerging side reference axis SA is reduced.
An excessive reduction in the angles ξ2 and ξ4, however, causes interference of luminous flux, so that these angles cannot be reduced significantly. In other words, the angles ξ1 and ξ3 need to be increased.
In general, excellent performance is difficult to achieve when the angle between the reference axis and a reflecting surface is increased. If a large angle is required between the incident side reference axis and the emerging side reference axis, the angles ξ1 and ξ3 are increased and thus a required level of performance cannot be provided. In addition, when the angle between incidence and emerging of light is large at a reflecting surface, the optical system is susceptible to holding errors.
FIGS. 18(A) and 18(B) show the influence of errors at different angles between incident and emerging of light at a reflecting surface. In FIGS. 18(A) and (B), A shows a light ray incident on the reflecting surface. FIG. 18(A) shows a large angle between incidence and emerging of the ray, while FIG. 18(B) shows a small angle between incidence and emerging of the ray.
A dotted line R represents the position of the reflecting surface based on designed values. A solid line R′ represents the position of the reflecting surface shifted by a length α from the position R to the right in FIGS. 18(A) and 18(B). β and γ show displacements of a hit point at which the light ray A hits the reflecting surface when the reflecting surface is shifted by the length α from the position R. As seen from FIGS. 18(A) and 18(B), the displacement β of the hit point at the large angle shown in FIG. 18(A) is larger than the displacement γ in FIG. 18(B) and demonstrates a greater influence on optical performance.
In this manner, even with the same position error of the reflecting surface, optical performance is more likely to degrade as the angle between incidence and emerging of the ray at the reflecting surface is larger.
FIG. 17(B) shows reflecting surfaces arranged to direct the emerging side reference axis SA upward. In this case, it is necessary to reduce angles ξ1 and ξ3 and increase angles ξ2 and ξ4. Similarly to the case shown in FIG. 17 (A), there is a limit to the extent of the reduction in the angles ξ1 and ξ3, and the angles ξ2 and ξ4 are inevitably increased more than necessary.
In this manner, simply increasing the tilt angle of the reflecting surface cannot readily increase the angle between the incident side reference axis and the emerging side reference axis. If the angle is larger than 30 degrees, a required level of performance cannot be ensured.